The present invention concerns night sights.
Sights have very widespread applications such as for aiming rifles and shotguns, telescopes and various surveying instruments and the like. Existing sights can be divided into two classes: telescopic sights which are practically small telescopes marked with a cross or a dot at the center of their field of view, and peep-hole sights, where one has to align a foresight and the target through a peep-hole. Telescopic sights have the advantage of magnifying the target without reducing its brightness. However, these sights are fragile and they have a narrow field of view which makes them impractical for heavy duty conditions. Therefore the most popular sights for heavy duty conditions, e.g. fire arms, is the peep-hole sight which is more useful because no fragile optics are required and the field of view is unlimited.
The accuracy of the peep-hole sight is simply: EQU .theta.=r/l (1)
where r is the radius of the peep-hole and l is the distance between the foresight and the peep-hole.
A telescopic sight can be adapted for use at night by providing for illumination of the center of the field of view. However, the same reasons for which a telescopic sight is not practical for heavy duty conditions at daytime it is also impractical at night.
Peep-hole sights can also be adapted for use at night by placing a point-like light source, e.g. a .beta.-light, at the center of the foresight. However, usually the peep-hole sight fails to work under conditions of bad illumination because the hole reduces the amount of light reaching the eye by the ratio (r/r.sub.i).sup.2, where r.sub.i is the radius of the pupil of the eye. The pupil of the eye adjusts its aperture according to the conditions of the illumination. In order to have good visibility at night the pupil of the eye widens and the ratio (r/r.sub.i).sup.2 decreases. A possible way to overcome this problem would be to replace the hole by a diaphragm iris of variable aperture. However, such devices often produce a non-circular aperture and besides, they are rather sensitive to humidity or dust. Moreover, high technical skills are required for adjusting the aperture of the sight to that of the eye.